Installation of downhole pumps in wells

ABSTRACT

A downhole pump and perforating gun are run simultaneously down a well on a tubing string and the gun is fired under underbalanced conditions. A slip joint reduces vertical mechanical shock transmitted through the tubing string to the pump on firing; centralizers minimize whiplash vibration and a lateral shock absorber absorbs radial shock.

This invention relates to a method of, and apparatus for, installing adownhole pump in a well such as, for example, an oil well in alow-pressure depleted reservoir or a new well with a low natural flowrate which is to be completed.

The present procedure for perforating an oil well is first to fire theperforators and then to kill the well to allow the pump(s) to belowered. Not only is this a cumbersome and time-consuming procedure, butkilling the well impairs the efficiency of perforators. It waspreviously thought impossible to run a downhole pump simultaneously witha tubing conveyed perforator because of the damage which would, it wasthought, be suffered by the pump.

Such simultaneous running is permitted by the present inventionaccording to a first aspect of which there is provided a method ofinstalling a downhole pump in a well comprising simultaneously running atubing conveyed perforator assembly and downhole pump, and firing theperforator assembly in underbalanced conditions while absorbing thetransmission of resultant mechanical shock through the tubing.

Preferably the perforator assembly and tubing are centralized during thefiring, and radial shock generated by the firing is absorbed. Preferablyagain the pump assembly is arranged at a nodal point of vibration of thetubing string.

According to a second aspect of the present invention there is provideda downhole pump and perforator assembly combination a tubing string, aperforator assembly mounted at the lower end thereof, a pump assemblysecured to the string at a position remote from the perforator assembly,and shock absorbing means for reducing vertical mechanical shocktransmitted through the tubing string. The reduction is preferablyachieved by a slip joint which allows relative upward movement of thelower portion of the string, thus forcing the fluid therein to bedisplaced.

Preferably the combination additionally comprises one or more of thefollowing components:

(1) Centralizing means to restrain the perforator assembly from rockingon being fired and inducing whiplash vibration in the tubing string;

(2) Centralizing means to minimize any whiplash vibration induced in thetubing string;

(3) Shock absorbing means for absorbing the radial shock generated byfiring the perforator assembly.

Preferably the impedance of the string to shock is increased byincreasing the mass thereof, e.g., by increasing the dimensions of theby-pass string to which the pump assembly is clamped and clamping theassembly solidly to the by-pass string.

Preferably, also, the pump assembly is located at a nodal point forvibration of the string achieved by maximizing the mass of a Y-crossoverlocated above the pump assembly, and of the pump assembly itself; bysolidly clamping the assembly to the tubing string as mentioned above;and additionally by minimizing the clearance between the outsidedimensions of the pump assembly and the internal diameter of the casing.

Moreover, the clearance of the perforator assembly with respect to theI/D of the casing should also be minimized.

By the use of the present invention the following advantages areafforded:

(1) The production index of the well is increased.

(2) The general benefits deriving from perforation with tubing conveyedperforating guns (which are bigger and better than conventional guns)are obtained, particularly

(a) Perforation under high underbalanced conditions with resultant good,clean perforations allowing maximum unrestricted flow from the reservoirthrough the tunnels into the casing, thus minimizing flow of fines anderoded materials through the pump, and

(b) Elimination of previous formation damage due to drilling orcompletion fluids.

(3) Rig time is saved by a reduction in the number of trips needed tocomplete the well.

(4) Production time is saved by elimination of the interval during whichthe well is killed after perforation.

(5) The tubing conveyed operation is simplified with respect toobtaining underbalanced and perforator detonation.

In practising the invention underbalanced must be achieved before theperforators are fired, preferably by using the pump, which allows itsoperation to be checked before perforation, and this method willsubsequently be described in more detail. However other methods may beused, such as running a partially full, closed string (suitable only fornew completions or work over wells with a very low natural flow rate) orpumping nitrogen down the annulus until enough volume has been displacedto obtain the desired underbalance.

When using the closed string method a special Y-tool is used to isolatethe annulus and tubing when running in hole; the tubing may be run dryor partially filled with fluid. After the packer has been set openingtool and detonating bar are lowered by wire line, the Y-tool is openedafter the wire line string is past the flow area, allowing fluid in theannulus to fill the empty tubing, thus obtaining underbalance.

The closed string method allows the well to be perforated inunderbalance even if the pump fails later, and the packer to be set by along string, saving a wire line trip; and if the Y-tool fails to open,the guns cannot be detonated. On the other hand the method entails thedisadvantage that the pump ca-not be checked before the perforators aredetonated.

The nitrogen method is useful if the pump fails after the pocket hasbeen set and it is desired to perforate the well before replacing thepump. To perform the method a sliding sleeve above the Y-tool is opened;nitrogen is pumped down the annulus until enough volume has beendisplaced to obtain the desired underbalance; the sliding sleeve isclosed, reverse flow being prevented by the provision of a check val e;and the perforating guns are detonated.

Although the invention may be carried out in a variety of ways, oneparticular embodiment thereof will now be described, by way of examplewith reference to the accompanying drawings in which

FIG. 1 is a diagrammatic, vertical section through a lower portion of awell showing a tubing conveyed perforator (tcp)/downhole pumpcombination according to the invention before firing;

FIG. 2 is a section simi ar to that of FIG. 1, after firing;

FIG. 3 is a horizontal section through the pump assembly shown in FIGS.1 and 2; and

FIG. 4 is a detail of the shock absorber shown in FIGS. 1 and 2.

A tcp/downhole pump combination is located within a well bore casing 10,broken at 12, and terminating in a 17.8 cm liner 14. The combination issuspended from a production string 16, having a terminal portion passingthrough a dual packer 18, and a interposed inclined connector 20 andcomprises a Y-crossover flow sub 19 from which depends a by-pass string22 and tubing joint 23 leading to a pump 24, the intake of which isprovided with a debris filtering screen 25, and tandem pump motors 26,27both of which are solidly clamped to the by-pass string 22 by clamps 28(see FIG. 4). The pump motors 26,27 are supplied by an electrical cable30 secured by cable clamps 32,33. A PSI unit 34 is secured to the lowerend of the motor 27.

The clearance between the pump assembly and the casing 10 is only 0.53cm to help to establish it as a nodal point during vibration of thestring.

Below the pump assembly is located a shock-absorbing slip joint 35 shownin more detail in FIG. 3 and comprising a hollow stem 36 whichterminates in a recessed flange 38 and over the body of which isslidably and rotatably received the upper end of a cylindrical housing40. To achieve a seal an O-ring 42 is received in the recess in theflange 38 and to absorb the shock of downward movement of the housing 40a shock ring 44 is interposed between the shoulder of the flange 38 andthe upper end of the housing 40. The ring 44 is made of a non-rubbermaterial to ensure that rubber fragments resulting from disintegrationof the ring are not drawn into the pump 24.

Suspended below the slip joint 35 by some 2,000 feet of 7.30 cm tubing46 (instead of the normal 6.03 cm) is a perforating gun assemblycomprising a gun release 48, pup joint 50, detonating head 52 andperforating guns 54. A tubing centralizer 56 is located between the pupjoint 50 and the liner 14 and further centralizer 58 between the pupjoint 50 and the liner 14.

Below the guns 54 is mounted a lateral shock absorber 60 to counteractrocking of the guns 54 induced by non-simultaneous detonation ofexplosive charges in the perforator guns.

To perforate the well the tcp/downhole pump combination is run down thewe 1 to the illustrated position. After setting the packer 18 and withboth the production string 16 and annulus between it and the casing 10full, the pump 24 is operated to lower the level of fluid in the annulasuntil the reading from the PSI unit 34 indicates -hat the hydrostaticpressure corresponding to the selected underbalance has been reached;the reading is confirmed by comparing it with the volume of fluid pumpedfrom the well.

If the natural flow rate of the well is sufficiently low the perforatingguns 54 may be detonated by dropping a bar from the surface. If thenatural flow rate is high, then the use of a slick line is recommended;by previously lowering the bar to a position near the detonating head52, the perforating guns 54 may be detonated immediately after theselected underbalance has been achieved and the pump 24 has beenswitched off.

The advantages afforded by the above procedure are:

(1) The pump 24 is tested before perforation is effected;

(2) The head of hydrostatic pressure in the tubing 16 is higher thanthat in the annulus, generating a reverse flow through the pump 24during the perforation operation, thus preventing solids from enteringthe pump 24;

(3) The well is allowed to flow under a decreasing drawdown as the levelof fluid in the annulus is raised by the contribution from the tubing16, permitting solids to drop to the bottom of the well.

The above method gives rise to the disadvantage that, before the pump 24may be switched back on again, the reverse flow through the pump 24should have ceased, otherwise the pump motors will be burned out: on theother hand waiting until flow through the pump 24 ceases can kill thewell. This disadvantage may be overcome by installing a check valve (notshown) just above the pump 24 which will prevent reverse flowtherethrough, and which will also prevent solids flowing through thepump 24. To ensure that the check valve is not leaking and causing themotors 26,27 to rotate, an ammeter may be installed at the surface tomonitor whether any current is being generated by the rotating motors.

An advantage of the use of a check valve is that the packer may be setby pumping down the string without having to run a blanking plug.

We claim:
 1. A method of installing a downhole pump in a well comprisingthe steps of running a tubing-conveyed perforator assembly, including aperforating gun, and a downhole pump down the well simultaneously bymeans of a tubing string, and firing the perforating gun inunderbalanced conditions while absorbing transmission of resultantmechanical shock through the tubing string.
 2. A method as claimed inclaim 1, wherein the perforator assembly and tubing string are centralduring the firing.
 3. A method as claimed in claim 1 wherein the radialshock generated by the firing is absorbed.
 4. A method as claimed inclaim 1 wherein the pump is arranged at a nodal point of vibration ofthe tubing string.
 5. A downhole pump and perforator assemblycombination comprising a tubing string, a perforator assembly includinga perforating gun mounted at the lower end of said tubing string, a pumpassembly secured to the tubing string at a position remote from theperforator assembly, and shock absorbing means interposed in a by-passstring connected to said tubing string for reducing vertical mechanicalshock transmitted through the tubing string.
 6. A combination as claimedin claim 5, in which said shock absorbing means comprise a slip jointwhich allows relative upward movement of the lower portion of the tubingstring with respect to the by-pass string, thus forcing the fluidtherein to be displaced.
 7. A combination claimed in claim 5, furthercomprising centralizing means for restraining the perforator assemblyfrom rocking on being fired and inducing whiplash vibration in thetubing string for minimizing any whiplash vibration induced in thetubing string.
 8. A combination as claimed in claim 5 further comprisingshock absorbing means for absorbing the radial shock generated by firingthe perforating gun.
 9. A combination as claimed in 5 in which the massof the tubing string is increased to increase its impedance to shock.10. A combination as claimed in claim 5 in which the pump assembly islocated at a nodal point for vibration of the tubing string.
 11. Amethod as claimed in claim 2, wherein the pump is arranged at a nodalpoint of vibration of the tubing string.
 12. A method as claimed inclaim 3, wherein the pump is arranged at a nodal point of vibration ofthe tubing string.
 13. A combination as claimed in claim 6, in which themass of the tubing string is increased to increase its impedance toshock.
 14. A combination as claimed in claim 7, in which the mass of thetubing string is increased to increase its impedance to shock.
 15. Acombination as claimed in claim 8, in which the mass of the tubingstring is increased to increase its impedance to shock.
 16. Acombination as claimed in claim 6, in which the pump assembly is locatedat a nodal point for vibration of the tubing string.
 17. A combinationas claimed in claim 7, in which the pump assembly is located at a nodalpoint for vibration of the tubing string.
 18. A combination as claimedin claim 8, in which the pump assembly is located at a nodal point forvibration of the tubing string.
 19. A combination as claimed in claim 9,in which the pump assembly is located at a nodal point for vibration ofthe tubing string.
 20. A combination as claimed in claim 13, in whichthe pump assembly is located at a nodal point for vibration of thetubing string.